Light emitting diodes (LEDs) have been used for decades in applications requiring relatively low-energy indicator lamps, numerical readouts, and the like. In recent years, however, the brightness and power of individual LEDs has increased substantially, resulting in the availability of 1 watt and 5 watt devices.
While small, LEDs exhibit a high efficacy and life expectancy as compared to traditional lighting products. For example, a typical incandescent bulb has an efficacy of 10-12 lumens per watt, and lasts for about 1000 to 2000 hours; a general fluorescent bulb has an efficacy of 40 to 80 lumens per watt, and lasts for 10000 to 20000 hours; a typical halogen bulb has an efficacy of 20 lumens and lasts for 2000 to 3000 hours. In contrast, red-orange LED can emit 55 lumens per watt with a life-expectancy of about 100,000 hours.
Notwithstanding recent advances in LED efficiency, and the promise of dramatic energy savings, known systems have failed to capitalize on the LED's desirable characteristics and produce systems that can replace standard lighting products used in the commercial and consumer realms. This is primarily due to the limitations inherent in currently known light engines.
For example, commercial high power LED devices generate an enormous amount of heat—on the order of about 50 W/cm2. In order to achieve reliability and long life, it is important to keep the temperature of the LED devices fairly low. Currently known systems have failed to assemble multiple LEDs in a compact fashion while maintaining the necessary heat transfer characteristics.
Similarly, it is desirable to protect the LED die with some form of coating, but it is difficult to reliably protect an array of multiple LED die using a standard semiconductor passivation as the thermal stresses resulting from temperature excursions (particularly in large scale assemblies) can caused sheared wire bonds, fractured die bonds, and other reliability problems.
Furthermore, efforts to incorporate multiple color LEDs to produce white light have been undesirable because, even when the LED devices are assembled in close proximity (which is again limited by heat transfer considerations), the light produced by such systems is not well mixed, resulting in uneven blotches of individual colors rather than uniform projection of white light. Similarly, current production compound semiconductor LED colors cannot produce certain wavelength efficiently (e.g., 575 nm yellow light). Mixing of efficient red and green LED light is a better approach.
Accordingly, there is a need for LED light engine devices that overcome these and other limitation of the prior art.